While talk heats up that Windows 8 is killing the PC market it turns out that a Windows 7 security patch may be literally knocking out some PCs. Microsoft is warning Windows 7 users to uninstall a security patch it issued earlier this week because some PCs won't restart after deploying the fix.

Specifically, Microsoft is advising users of both Windows 7 and Windows Server 2008 R2 to revert to the pre-patch state. These users should uninstall the patch in security bulletin MS 13-036. MS13-036 is another kernel mode drivers issue, similar to the other kernel issue this month.

"We've determined that the update, when paired with certain third-party software, can cause system errors," said Dustin Childs, group manager of Response Communications for Microsoft Trustworthy Computing. "As a precaution, we stopped pushing 2823324 as an update when we began investigating the error reports, and have since removed it from the download center."

Setting the Record Straight

Childs then moved to set the record straight. Contrary to some reports, he said, the system errors do not result in any data loss nor affect all Windows customers. However, he added, all customers should follow the guidance Microsoft has provided to uninstall the security update if it is already installed. That information can be found at support.microsoft.com/kb/2839011.

"[The patch] addresses a moderate-level vulnerability that requires an attacker to have physical computer access to exploit," Childs said. "MS13-036 remains available for download and is being pushed via updates to help protect customers against the other issues documented in the security bulletin -- it no longer contains the affected update."

What are the other issues? We asked Paul Henry, a forensics analyst at Lumension, for a rundown on the threats. He told us there are four Common Vulnerabilities and Exposures (CVE) in MS13-036.

"Three allow a local user to use kernel-raise conditions to elevate to system access. The fourth CVE is a moderate elevation of privilege issue, which is unusual for Microsoft. To leverage this CVE, an attacker would need to be an admin, which removes the need to leverage it," Henry said.

"Alternatively a low rights user would need to use a specially crafted external device, such as a USB. Last month, Microsoft had an interesting USB bug that got a lot of attention. This is nothing like that. Last month's bug allowed computers to be attacked regardless of the user's log-in status. This month's bug only allows a logged on, active system to be attacked, so log-on credentials are required. There are easier ways for an attacker to get in."

Pity on Microsoft?

Ken Pickering, development manager of security intelligence at Core Security, said that coming from an engineering background, he feels sorry for Microsoft. That's because many people don't realize that code fixes can be even more destructive than the original problem.

"That's why we invest so much time in QA to make sure the fix isn't actually worse than the problem. Unfortunately for Microsoft, it looks like they fell a bit short of this benchmark, and introduced an even more destructive fix than the 'moderate' privilege escalation bug they were trying to remediate," Pickering said.

"An even funnier statistic is how many potential security holes a security product can introduce versus how many would exist if the product was not running. Looking at Core's library of exploits against established security products or processes in Windows that are supposed to make the system more secure is humbling."

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